MgADP Promotes a Catch-Like State Developed through Force-Calcium Hysteresis in Tonic Smooth Muscle

Abstract

Tonic rabbit femoral artery and phasic rabbit ileum smooth muscles permeabilized with Triton X-100 were activated either by increasing [Ca 2+] from pCa > 8.0 to pCa 6.0 (calcium-ascending protocol) or contracted at pCa 6.0 before lowering [Ca 2+] (calcium-descending protocol). The effects of, respectively, high [MgATP]/low [MgADP] [10 mM MgATP + creatine phosphate (CP) + creatine kinase (CK)] or low [MgATP]/[MgADP] (2 mM MgATP, 0 CP, 0 CK) on the “force-[Ca]” relationships were determined. In femoral artery at low, but not at high, [MgATP]/[MgADP] the force and the ratio of stiffness/force at pCa 7.2 were significantly higher under the calcium-descending than calcium-ascending protocols (54% vs. 3% of P o, the force at pCa 6.0) (force hysteresis); the levels of regulatory myosin light chain (MLC 20) phosphorylation (9 ± 2% vs. 10 ± 2%) and the velocities of unloaded shortening V 0 (0.02 ± 0.004 l/s with both protocols) were not significantly different. No significant force hysteresis was detected in rabbit ileum under either of these experimental conditions. [MgADP], measured in extracts of permeabilized femoral artery strips by two methods, was 130–140 μM during maintained force under the calcium-descending protocol. Exogenous CP (10 mM) applied during the descending protocol reduced endogenous [MgADP] to 46 ± 10 μM and abolished force hysteresis: residual force at low [Ca 2+] was 17 ± 5% of maximal force. We conclude that the proportion of force-generating nonphosphorylated (AM dp) relative to phosphorylated cross-bridges is higher on the Ca 2+-descending than on the Ca 2+-ascending force curve in tonic smooth muscle, that this population of positively strained dephosphorylated cross-bridges has a high affinity for MgADP, and that the dephosphorylated AM dp · MgADP state makes a significant contribution to force maintenance at low levels of MLC 20 phosphorylation.